1. Field of the Invention
The present invention is directed, in general, to the field of DC-to-DC converters. More particularly, the present invention relates to a multiple-output flyback converter having an improved energy cycle sequencing capability.
2. Description of The Related Art
Conventional flyback switching power supplies commonly include a pair of transformers (actually a pair of coupled inductors) and one or more power switches for alternately coupling an unregulated DC or rectified AC voltage across a primary winding of the power transformer in a series of voltage pulses. These pulses are converted into a series of voltage pulses across one or more secondary windings of the power transformer and then rectified and filtered to provide one or more output DC voltages. The output voltage or voltages of the power converter are commonly regulated by controlling the relative amount of time that the power switch is xe2x80x98onxe2x80x99 (i.e., the duty cycle).
One common type of switching power supply is the flyback power converter, which is an isolated version of the buck-boost converter. The flyback converter is a very popular power supply topology for use in low-power, multiple-output applications. A flyback power converter works by cyclically storing energy in the power transformer, and then dumping this stored energy into a load. By varying the amount of energy stored and dumped per cycle, the output power can be controlled and regulated. A high-power switching transistor connected in series with the primary winding of the power transformer normally provides such a switching function. That is, the on-time and off-time of this power switch controls the amount of energy coupled across the power transformer. When the power switch is xe2x80x98onxe2x80x99, current flows through the primary winding of the power transformer, and energy is stored in the transformer (primary magnetizing inductance). When the power switch is xe2x80x98offxe2x80x99, the stored energy is transferred out into a secondary circuit by means of current flowing out of one or more secondary windings of the power transformer. Note that the secondary current does not flow in the power transformer at the same time that the power switch is xe2x80x98onxe2x80x99 and the primary current is flowing. The reason for this is that in a conventional flyback power converter, the winding polarity is chosen and a rectifier is coupled to the secondary winding to prevent conduction of current in the secondary winding when the power switch is xe2x80x98onxe2x80x99.
Flyback power converters are advantageous at lower power levels over other switching power converters due to the fact that they are generally simpler, they require a reduced number of components, and they allow multiple regulated outputs to be available from a single supply. Common applications for flyback converters are AC adapters, which may, for example, deliver an output voltage in the range of between 9 VDC to 180 VDC at power levels of 20 to 100 Watts, drawing power from a rectified AC line voltage, which may vary between 85 VAC to 270 VAC for universal line voltage inputs.
Flyback converters are generally operated in one of two modes. A first mode of operation, referred to as the Discontinuous Conduction Mode (DCM), is well known in the art, in which the energy stored in the transformer is totally coupled to the output load before the next energy cycle, generally resulting in the secondary current reaching zero before the next drive cycle. The second mode of operation is referred to as the Continuous Conduction Mode (CCM), whereby the next energy cycle begins before all stored magnetic energy is released from the transformer, and, therefore, before the secondary current reaches zero. DCM is more common than CCM because relatively simple control circuitry can be used to maintain output voltage regulation by varying the frequency and/or on-time of the power switch to accommodate heavy or light load conditions.
FIG. 1 illustrates a conventional flyback converter 100 which may be operated in either DCM or CCM. One disadvantage of the flyback converter 100 is that when operated in the CCM, the flyback converter 100 may exhibit potentially unstable operation when used with high bandwidth feedback loops. That is, the converter 100 is susceptible to oscillations when high bandwidth feedback loops are used. Another disadvantage of this circuit topology is that the diode D1 is hard-switched. That is, in the CCM, current is reversed while the diode D1 is still conducting.
FIG. 2 is an illustration of another conventional flyback DC-DC converter whose topology is similar to that illustrated in FIG. 1 except for diode D1 being replaced by switch S1. In the circuit of FIG. 2, the body diode of switch S1 provides the same functionality as the diode D1 of the circuit of FIG. 1. The circuit of FIG. 2 functions in the same manner as the circuit of FIG. 1 when switch S1 is held xe2x80x98offxe2x80x99. However, when switch S1 is turned xe2x80x98onxe2x80x99, bi-directional current flow is enabled. That is, current can flow in the reverse direction (i.e., out of the filter capacitor C1 through the secondary winding, nS1).
FIG. 3a is an illustration of another prior art flyback converter 310 having multiple output circuits 311, 313. Each of the respective output circuits 311, 313 includes a blocking diode (D1, D2) and a unidirectional switch (S1, S2). If the unidirectional switch S1 is in the xe2x80x98offxe2x80x99 state, then current is blocked from flowing into the output. Therefore, the xe2x80x98onxe2x80x99 time of switch S1 controls the power flow to the output of circuit 311.
A drawback of circuit 310 is that when the circuit operates in the discontinuous conduction mode, it is necessary to drain the transformer T of energy in each cycle. This may be problematic in that the unidirectional switch associated with the last or final output circuit in the switching cycle (e.g., switch S2) has to remain xe2x80x98onxe2x80x99 for a long enough time to fully drain the transformer T in each switching cycle. To ensure that this occurs, in actual practice, switch S2 is often eliminated.
FIG. 3b is an illustration of the circuit of FIG. 3a which eliminates circuit switch S2. As discussed above, this ensures that the transformer T will be fully drained in each switching cycle. Elimination of switch S2 causes output V2 to be controlled solely by the xe2x80x98onxe2x80x99 time of the primary switch SM.
FIGS. 3a and 3b illustrate circuits having two output circuits. Irrespective of the number of output circuits, however, primary side control of the final output circuit occurs. In each case, in DCM, the last output circuit would be controlled by the xe2x80x98onxe2x80x99 time of the primary switch SM. It may be desirable, in certain cases, to retain the output circuit switch in the last output circuit even though the output is effectively controlled by the xe2x80x98onxe2x80x99 time of the primary switch. This is true because retaining the switch enables the implementation of synchronous rectification and primary soft-switching.
FIG. 4 is an illustration of another prior art circuit topology, which is a modification of the circuit topology of FIG. 3b. The circuit of FIG. 4 includes a switch S2 in place of blocking diode D2 of the circuit of FIG. 3b. The circuit topology of FIG. 4 provides advantages over the topologies of FIG. 3b in that the bi-directional switch S2 permits synchronous rectification and further permits bi-directional soft switching of primary switch SM. However, even though the illustrative prior art topology of FIG. 4 enables synchronous rectification and soft switching, it shares the drawback or restriction, common to all the prior art circuits illustrated, in that the last output (e.g., output V2 in FIG. 4) has to be cycled last in each energy cycle. From a control or circuit standpoint, this restriction may not always be desirable.
This restriction exists by virtue of the circuit topology. In particular, the cycling restriction exists because switch S2 of output circuit 413 has no forward blocking capability. As a consequence of the lack of forward blocking capability, voltage V2 of output circuit 413 must be maintained at a higher voltage than V1 so as to reverse bias the body diode of switch S2 to compensate for the lack of forward blocking capability. By contrast, output circuit 411 provides a forward blocking capability and can therefore be cycled at any point in the cycling sequence. The forward blocking capability is provided by the internal body diode of switch S1.
It is therefore an object of the present invention to provide an improved multiple output power converter capable of providing soft-switching and being further capable of sequencing the outputs in any order without restriction.
A primary objective of this invention is to provide a DC-to-DC flyback converter having an easily reconfigurable output cycle sequencing capability.
Yet another objective is to provide a DC-to-DC flyback converter which enables simultaneous soft switching of primary and secondary side switches.
A still further objective is to provide a DC-to-DC flyback converter which avoids diode drop losses in low voltage, high current supplies.
Another objective is to provide a DC-to-DC flyback converter having an easily reconfigurable circuit configuration for applications involving power down.
A still further objective is to implement the secondary side switches as current bi-directional switches, such as MOSFET""S, thus providing an output cycling reconfiguration capability.
These and other objects of the invention are achieved in a multiple-output flyback converter. The flyback converter circuit of the present invention includes multiple output circuits, each having a bi-directional switch which provides for flexible reconfiguration of the output cycling sequence. Further, the novel circuit allows each of the individual outputs, or any combination thereof, to be independently turned xe2x80x98offxe2x80x99 (i.e., removed from the cycling sequence) and re-introduced (i.e., turned xe2x80x98onxe2x80x99 again) at a later time as needed.
The flyback converter circuit according to the present invention includes a power transformer, a primary side switch and multiple output channels or circuits, each output circuit including a voltage controlled bi-directional switch permitting each output channel to be driven independently and, therefore, capable of being cycled in any order during a time the primary switch is turned xe2x80x98offxe2x80x99. By providing a bi-directional switch in each output channel, any restrictions on the sequencing of the respective output channels are removed. That is, each output channel can be cycled in any order with limited restrictions while still maintaining synchronous rectification and soft-switching available with bi-directional action.
Advantages of the present invention include simultaneous soft-switching of the primary-side and secondary-side switches, and higher circuit efficiency as a consequence of using bi-directional switches in the output circuits instead of diodes resulting in lower circuit losses especially in low-voltage, high current applications.